The present invention is related to a process for reducing metal oxide containing ores.
The process of the present invention seeks to conduct the said reduction process under very favorable kinetic conditions, at practically instantaneous speed. It is an object of the invention to obtain metallic particles as a final product of the reduction substantially free from agglomerations resulting from the partial sintering of the mineral particles before the reduction.
The known processes for reducing metal oxides in minerals or ores are based on the direct action of reducing agents (except for hydro metallurgical and electrolytic processes) after having exposed previously prepared ore pieces of the proper size, according to the process to be used, to the action of reducing agents (generally carbon, monoxide, hydrogen or a mixture thereof), in such a manner that the reducing agents or their products of reaction are contacted with the mineral, at a temperature adequate for the occurrence of the reduction reaction and for the time necessary for its completion.
Taking as an example the reduction of minerals containing iron oxides, the conventional procedures for processing those containing hematite (Fe.sub.2 O.sub.3) or magnetite (Fe.sub.3 O.sub.4) employ any one of the devices: shaft furnace, rotary kiln, fluidized bed, or travelling grate.
A blast furnace is a shaft furnace of great dimensions, through the upper part of which iron ore is fed in pieces of variable size, but containing limited quantitites of sizes smaller than 12,7 mm (1/2") and alternating the ore charges with others composed of carbon in the form of coke and other elements destined to form the slag.
The carbon supplies the reducing agent in the form of carbon monoxide through partial oxidation thereof by the air blown through nozzles at the bottom of the furnace. The oxidation raction also supplies the required heat to attain the temperature of reduction.
The exposure of the ore to the reducing agent at high temperature reduces the iron oxides and produces the fusion of metal and gangue, which are tapped from the lower part of the furnace.
The so-called "direct reduction" processes known as the Midrex, Purofer and Armco processes also use vertical shaft furnaces. The Hyl (Hojalata y Lamina) makes use of vertical reactors in a batch system.
All of these processes use gaseous reducing agents (hydrogen, carbon monoxide or a mixture of both gases obtained from cracking natural gas or other liquid or gaseous hydrocarbons or from the gasification or distillation of coal). These gases, heated at adequate temperature, circulate through the ores in countercurrent fashion, while the latter slowly descend through the shaft furnace (or, in the case of the Hyl process, while the mineral stands in the reactor).
All of these processes require that the ores to be treated be of a relatively large size, or also be previously agglomerated in the form of pellets or briquets.
Procedures using a rotary kiln use solid coal as a reducing agent. The coal is mixed with the ores prior to entering the rotary kiln, or it is projected from the discharge outlet towards different points inside the kiln. As in the case of the blast furnace, the coal supplies the reducing agent (CO) and the reaction heat derived from its oxidation maintains the temperature for reducing the mineral.
The iron ore must be fed in pieces ranging in size from 2 to 20 mm or in the form of pellets from 10 to 15 mm.
Procedures using fluidized beds employ gaseous reductants. The mineral, ground in small and preferably uniformly sized pieces (about 150 to 300 microns), is submitted to the action of ascending reducing gases, the speed of which is such that the particles without being entrained are suspended, and the mineral mass behaves like a fluid.
In procedures using a movable grate the ground mineral is agglomerated (pelletized) together with coal fines and the green pellets conveyed on a circular movable grate while the reducing gas heated to the proper temperature for preheating is passed through the bed of pellets. In successive areas of the circuit the coal reacts with the oxygen in the mineral to reduce the latter followed by cooling.
Of prior art processes and devices used for reducing iron oxide containing minerals, certain general conclusions are self-evident.